1. Field of the Invention
The present invention relates to rod-type or cylindrical insulators, such as, long-rod insulators for transmission line, station post insulators for power transformation and bushing shells.
2. Description of the Prior Art
The surface of insulators used with electric power equipment for electrical insulation purposes is subject to contamination with electrolytes, such as, sea salt, industrial contaminants and the like. Consequently the dielectric strength of the leakage surface of such a contaminated insulator will be decreased if it is subjected to wetting by rain, fog, mist or the like. As a result, much importance has been attached to the withstanding voltage characteristics under a contaminated condition of an insulator in designing electric power equipment. The withstand voltage under a contaminated condition of a rod-type insulator is affected considerably by the leakage distance along the insulator surface and it is well known in the art that the withstand voltage under a contaminated condition can be increased by several structural modifications including increasing the height of the insulator, increasing the number of sheds without changing the height of the insulator, increasing the number of ribs or providing the trunk portion with ribs so as to increase the leakage distance along the insulator surface. However, to increase the height of the insulator is disadvantageous from the standpoints of mechanical designing and economics since it increases the size of the instrument as a whole. Increasing the leakage distance along the surface by simply increasing the number of sheds or the number of ribs or corrugations without changing the height of the insulator cannot proportionately increase the withstand voltage under contaminated condition, and the withstand voltage is greatly affected by the shape of the insulator sheds including the shed pitch and other conditions. This shed shape is the most important factor that must be considered particularly in anti-contamination design of insulators and bushing shells having a long leakage distance. Thus, a prior art rod-type insulator of the type shown in FIG. 4 of the accompanying drawings has been designed which takes into consideration the ratio of the leakage distance L per shed 2 to the shed pitch P of an insulator body 1 or L/P. With this prior art insulator, however, no consideration has been given to the maximum possible inter-shed coefficient which will be defined as l/p, where p represents the distance between the adjoining insulator sheds, i.e., the distance between a given point B on the lower surface of one shed 2 and another given point C on the upper surface of the other opposing shed 2 and l represents the leakage distance between the given points B and C along the insulator surface. A phenomenon has been observed in these prior art insulators in which an arc bridges across a rib or corrugation 3 on the lower surface of one shed 2 and the upper surface of the other opposing shed 2. This arcing occurs because the inter-sheds coefficient is too large thus preventing effective functioning of the leakage distance L of that portion. As a result, if the leakage distance L of the insulator is simply increased, the withstand voltage under a contaminated condition cannot be increased correspondingly. Where the length of the insulator is fixed there exists from the standpoint of insulator design a long felt need for the determination of a shed shape which ensures an excellent insulator with the highest possible withstand voltage under a contaminated condition.